1. Field of the Invention
The invention relates to optical data storage systems, and more particularly, to an optical recording apparatus capable of adjusting a write defocus value of optical signals used to write data on an optical medium.
2. Description of the Prior Art
In the field of optical data storage systems, it is necessary to read data from and write data to an optical medium utilizing an optical recording apparatus. The optical recording apparatus detects variations or marks on the optical medium (e.g. an optical disc). These marks are written to the optical medium in a meaningful manner and make up the stored data. Therefore, accurate creation and detection of these marks on the optical medium is essential in order to reliably store and reproduce data.
The optical recording apparatus typically includes various lenses and positioning devices which cooperate with one another to provide focusing on the optical media. The optical characteristics of this system will often change over time and thus cause variations in the system operation. These changes can result from a large number of things such as changes in temperature causing related changes in optical properties, all of which dramatically affect the operation of the system. Further, the optical elements are typically mechanically positioned and aligned. This alignment and positioning can vary as the optical recording apparatus itself encounters shock and vibration. All of these misalignments affect the focusing of the read/write system, and ultimately affect the performance of the disk drive.
Typically, read out systems include mechanisms which allow some offset to be injected into the focusing control. Often, focusing is controlled by a servo which will drive a focus motor and appropriately position the focusing lens. As is well known, an offset can easily be introduced into this servo system in order to vary the focus operations. This focusing offset is often referred to as a defocus value. Due to the circumstances outlined above, it is necessary to periodically check the focusing operation and insure that optimum focusing parameters such as the defocus value are being used.
In addition to variations in the alignment of the focusing system, the actual storage media may not be consistent. Substrate thicknesses in the optical media often change from disc to disc. Therefore, the read defocus is typically optimized every time an optical medium such as an optical disc is changed.
Various methodologies have been used to optimize the read defocus value, and much development has been performed to develop a system and process for efficiently optimizing the read defocus of an optical storage system. For example, U.S. Pat. No. 5,574,706 entitled “Focus Offset Optimization For Reading Optically-Recorded Data” describes one such methodology wherein both control marks and reference data marks are recorded in each frame when the disk is initially written. The recorded reference data marks are placed no farther apart than the smallest distance between two successive data marks which must be resolved as a blank space or zero during the read out process. Analyzing the signals produced when reading out these signals, and insuring that the distance between marks can be resolved allows for optimum read focus. Other systems have used isolated data sectors which are continually read at different read focus offset levels.
During write operations of an optical recording apparatus, the defocus of optical signals used to write data to the optical medium must also be set to a particular value. In the related art, this write defocus value is simply set equal to the optimum read defocus value. However, the read defocus value may not be optimal for write operations having higher laser powers and different characteristics. Therefore, degradation in the accuracy of the stored data may occur when writing data to an optical medium while using the optimal read defocus value.